Method of producing composite structures of ultra-high-molecular-weight polymers, such as ultra-high-molecular-weight polyethylene products

ABSTRACT

Method of producing a single-phase composite structure of filamentary and non-filamentary semicrystalline morphology made from the same polymer, which is of a type capable of gelling in a suitable solvent and of being deformed into a high-modulus, high-strength product. Layers of the polymer in sheet form are interleaved with at least one layer, also of that polymer, made from filaments thereof. The method of making the product may involve heating a sheet of UHMWPE or other polymer gel (5% UHMWPE in 95% paraffin oil, by weight) to 125° C., applying a knitted UHMWPE high modulus, high-strength structure on one side thereof, extracting the non-volatile paraffin oil therefrom with hexane, and evaporating the hexane.

REFERENCE TO RELATED PATENT APPLICATION

This application is a division of patent application Ser. No. 262,970,filed Oct. 26, 1988, now U.S. Pat. No. 4,944,974, which was acontinuation-in-part of patent application Ser. No. 132,200, filed Dec.14, 1987, now abandoned which was a continuation-in-part of patentapplication Ser. No. 936,838 filed Dec. 2, 1986, now abandoned, whichwas a continuation-in-part of patent application Ser. No. 811,015, filedDec. 18, 1985, now U.S. Pat. No. 4,655,769, to which reference may bemade as to details described there.

This invention relates to composite structures ofultra-high-molecular-weight polymers, as for example, polyethylene(UHMWPE) composites, of both filamentary and non-filamentarysemicrystalline morphologies. In the composite structure of thisinvention the matrix and its fiber reinforcement are comprised of thesame polymer resin, as for example, an ultra-high-molecular weightpolyethylene, or of polymer resins with similar properties. Suchcomposite structures are prepared by a processing methodology thatincludes the gelation of the matrix components on the reinforcingfibrillar component and the subsequent extraction or evaporation of thesolvent that is incorporated initially in the matrix from the composite.

BACKGROUND OF THE INVENTION

The parent application, now U.S. Pat. No. 4,655,769, describes andclaims a pseudo-gel comprising a suitable solvent in an amount of 99 to90 percent by weight and an ultra-high-molecular-weight polymer such aspolyethylene (UHMWPE) in an amount of 1 to 10 percent by weight Thepseudo-gel of the ultra-high-molecular weight polymer, e.g. anultra-high-molecular-weight polyethylene, is a semicrystalline networkwith adjustable crystalline morphology comprising randomly dispersed andoriented chain-folded single crystals, stacks of single crystals,spherulitic crystals, and extended-chain shish-kebab-type of fibrilswith lengths up to a few millimeters and widths up to 20 μm. Thesemicrystalline ultra-high-molecularweight polymer, such aspolyethylene, is obtained by removal of the solvent from the pseudo-gel.The application also describes a method for making a pseudo-gelprecursor and a ultra-high-molecular-weight polyethylene product.

Near the end of that application, reference is made to compositestructures of UHMWPE filamentary and non-filamentary semicrystallinemorphologies. The present invention relates to such composite structuresand to the processing methodology relating thereto.

FABRICATING COMPOSITES

The fabrication of composites from the same polymer, that is, compositesin which both the matrix and the reinforcing fibers are comprised of thesame polymer, e.g., polyethylene, has been possible both when usingmatrix and another as a reinforcing fiber, and also when using differenttypes of polyethylene, one as a matrix and another type as a reinforcingfiber. In either case, the underlying principle is that the polyethylenematrix component is transcrystallized from the melt state on thepolyethylene reinforcing fiber; this process is critically dependent onthe difference of the melting temperatures of the polyethylene matrixcomponent and the polyethylene reinforcing fiber component of the samepolymer composite system. Thus, for example, for high densitypolyethylene with weight-average molecular weight of about 60,000, themelting point of the unprocessed (as received) resin pellets is about132° C. and the melting point of the highmodulus and high-strengthfibers of the same polymer resin produced by solid state deformation isabout 138° C. This melting point difference of only about 6° has beenutilized for the fabrication of a single-phase composite, but it is fartoo small for the practical and commercial fabrication of a single phasecomposite of this polyethylene with high tensile modulus andhigh-strength performance using this particular methodology, i.e., bytranscrystallization from the melt state on to the polyethylenereinforcing fiber because of the partial melting of the fibers duringthe transcrystallization of the matrix on them, which resulted in thedeterioration of their properties and consequently of the properties ofthe composite. One way to alleviate this processing difficulty is to usepolyethylenes of different types, i.e., a lowdensity polyethylene or alow or high density polyethylene copolymer as a matrix component and ahigh-density polyethylene as a reinforcing fiber component with theirrespective melting points significantly different, so that thetranscrystallization of the polyethylene matrix component from the meltstare can occur without the deterioration of the mechanical propertiesof the polyethylene fiber component. Prior art shows the use of thisapproach with the fabrication of a composite of a low densitypolyethylene as a matrix and a high density polyethylene as thereinforcing fiber. However, these polyethylenes have physical andmechanical properties different from each other, in particular, meltingtemperatures, as the melting temperature of the low density polyethyleneis 98°-115° C. and of the high density polyethylene 130°-137° C. Alsothe low density polyethylene is a branched polymer whereas the highdensity polyethylene is a linear polymer. For example, U.S. Pat. No.4,457,785 shows the molding of low-density polyethylene and high-densitypolyethylene fibers into a polyethylene composite structure. Also, thesame patent shows the molding of a copolymer of high-densitypolyethylene and ethylene hexane-1 with high-density polyethylenefibers. Similarly, a composite of a single polymer can be prepared byusing a matrix of high-density polyethylene having a weight averagemolecular weight of (e.g.) 60,000 and a melting temperature of about130° C. to 138° C. and a reinforcing fiber component ofultra-high-molecular-weight polyethylene (e.g., M_(w) ˜3 × 10⁶) with amelting temperature of about 142° C., by transcrystallizing thelow-molecular weight polyethylene matrix component from the melt ontothe fibers; but again such a single-polymer composite is composed ofvery different types of polyethylenes, and it is formed bytranscrystallizing the low-molecular weight polyethylene from the meltstate onto the ultra-high-molecular weight polyethylene fibers. Thelow-molecular-weight polyethylene is readily meltprocessable, whereasfor all practical purposes the ultra-high-molecular-weight does not havea measurable melt-flow index. The very high molecular weight (severalmillion) of the ultra-high-molecular-weight polyethylene gives it uniqueproperties and processing characteristics which are not encountered inthe normal high-density polyethylenes (e.g., high-density polyethyleneresins with molecular weights up to about 300,000-400,000). Thus thepreparation of a composite structure by the process of meltcrystallizing the matrix on the reinforcing fibers using the same UHMWPEresin as a matrix and fiber (e.g., an UHMWPE resin with weight averagemolecular weight greater than 3 × 10⁶) is not possible for two basicreasons:

(1) The UHMWPE at its melting temperature (T_(m) ˜ 38° ) does not flowto coat the UHMWPE fibers as it would have happened with a high-densitypolyethylene resin having a low weight-average molecular weight, e.g.,Mw 60,000-300,000, and

(2) the melting temperature of the UHMWPE in the matrix (T_(m) ˜138° C.)and fiber (T_(m) ˜142° C.) form are very close, and the use of themelt-crystallization approach leads to the deterioration of themechanical properties of the reinforcing fibers because of their partialor complete melting. Therefore, it is apparent that the prior artteaches that single-phase composites can be fabricated only by usingdissimilar resins and that such composites are comprised of amelt-crystallized matrix into the reinforcing fibers In addition, itbecomes important to notice that unless the polyethylenes used as thematrix and those used as the reinforcing fiber are of different typesand have melting points that are significantly different, thefabrication of single-phase composites in which the matrix and thereinforcing fiber are composed of the same polymer resin or from polymerresins with identical or similar properties prior to their processinginto the matrix and fiber forms, is impractical when using the teachingsof the prior art.

Thus, an object of this invention is the fabrication of a single-phasecomposite without the transcrystallization of the matrix from the melton the reinforcing fibers, by using a methodology which involves thegelation of the matrix component on the reinforcing fiber component andthe subsequent removal of the solvent from the matrix (by extraction orevaporation, depending on the volatility of the solvent) while it isincorporated into the composite structure.

A key issue in the use of this methodology, shown clearly in the parentpatent (U.S. Pat. No. 4,655,769) is that the pseudo gel of anultra-high-molecular-weight polyethylene has a melting temperature(T_(m) ˜123° C.) which is substantially lower than the semicrystallinemorphology which is produced after the removal of the solvent; as shownin Table I in Col. 7 of the parent patent, the melting temperature ofthe semicrystalline UHMWPE morphology is ˜137° C. i.e., ˜14° C. greaterfrom the Tm of the pseudo-gel in paraffin oil (T_(m) ˜123°). Consideringthat the melting temperature of the highly oriented fibers made of thesame UHMWPE resin is ˜142°-145° C., there is a temperature difference of˜20° C. between the T_(m) of the pseudo-gel of UHMWPE in paraffin oiland the UHMWPE fibers for forming a single phase composite by gelling apolymer resin onto itself in another state, i.e., as fiber. Thissubstantial temperature drop of the melting temperature of pseudo-gelstate has the important implication that one can take any polymer resinthat is capable of gelling and apply it at the melting temperature ofthe crystals in the gel as matrix on a fiber of the same polymer resinor of the same polymer. The term pseudo-gel refers to a concentratedsolution of organic polymer which contains an entangledthree-dimensional semicrystalline network the morphology of which mayvary with the conditions of preparation or crystallization.

Polymer resins which can be used with the methodology of this disclosuremust be capable of (a) forming a pseudo-gel state as described in theparent Pat. No. 4,655,769, and (b) being deformed into high-modulushigh-strength fibrous products.

Polymers which meet these requirements must be linear and have a veryhigh molecular weight, such as ultra-high-molecular-weight polyethylene(UHMWPE), or polar groups in the chain backbone, such as the polyamides.

By polymer having a very high molecular weight, it is meant, a polymerresin having a weight average molecular weight of at least 500,000 andpreferably above two million.

Another object of this invention is the fabrication of a single phasecomposite in which the matrix has a different crystalline morphologyfrom the melt crystallized morphology of the matrix components used inprior art (references--U.S. Pat. Nos. 4,737,402; 4,457,985; 4,403,012).

Another object of this invention is the fabrication of a single-phasecomposite structure in which the matrix and the reinforcing fibers areof the same polymer resin.

Another object of this invention is the fabrication of a single-phasecomposite in which the matrix and the reinforcing fibers are from resinswith identical or similar properties prior to their processing into thematrix and the reinforcing fiber.

For example, a system of a matrix and reinforcing fibers of the sameUHMWPE resin, e.g., an UHMWPE with weight average molecular weight>3×10⁶, is a single phase composite falling under the scope of thisinvention because

(a) the matrix and the reinforcing fibers comprising the composite areproducts of the same UHMWPE polymer resin obtained by independentfabrication processes. If UHMWPEs of different manufacturers are used,component, such a composite system falls also under the scope of thisinvention because UHMWPE as defined by ASTM are those "linearpolyethylenes which have a relative viscosity of 2.3 or greater, at asolution concentration of 0.05% at 135° C.decahydronaphthalene"(Decalin), i.e., having a weight-average molecularweight of at least 3.1 × 10⁶ ;

(b) the UHMWPE forms a pseudo-gel state in a solvent such as paraffinoil or decahydronaphthalene (the capability to form a pseudo-gelassociates with the high molecular weight of the UHMWPE);

(c) UHMWPE is a linear polymer; and

(d) UHMWPE can be fabricated into a high-modulus and high-strengthfibrous product.

Similarly, the single phase composites in which the matrix and thereinforcing fibers might be composed of UHMWPE resins of differentmanufacturers, and single-phase composites in which the matrix and thereinforcing fiber components are comprised of linear polymer resins withthe same chemical structure and identical or similar properties prior totheir fabrication into a matrix and reinforcing fibers, are included inthe scope of the invention.

Other polymers which can be used under the scope of the inventioninclude polymers such as isotactic polypropylene, poly(L-lactide),poly(vinyl alcohol), polyacrylonitrile, poly(ethylene terephthalate) andpolyamides Solvents which may be used for dissolving and formingpseudo-gels with these polymers are Decalin (decahydronaphthalene) andparaffin oil for polypropylene, chloroform for poly(L-lactide), ethyleneglycol and water for poly(vinyl alcohol), dimethyl formamide andtetramethylene sulfone for polyacrylonitrile, nitrobenzene forpoly(ethylene terephthalate, and benzyl alcohol for the polyamides.

SUMMARY OF THE INVENTION

A single-phase composite according to this invention is one in which thematrix and the reinforcing fibers are of the same polymer resin or frompolymer resins with identical or similar properties prior to theirprocessing into the matrix and fiber components of the composite.

Polymers which can be used in the invention include polyethylene andpolymers such as isotactic polypropylene, poly(L-lactide), poly(vinylalcohol), polyacrylonitrile, poly(ethylene terephthalate), andpolyamides. Solvents which may be used for dissolving and formingpseudo-gels with these polymers are Decalin and paraffin oil forpolypropylene, chloroform for poly(L-lactide), ethylene glycol and waterfor poly(vinyl alcohol), dimethyl formamide and tetramethylene sulfonefor polyacrylonitrile, nitrobenzene for poly(ethylene terephthalate, andbenzyl alcohol for the polyamides.

The feasibility of fabricating a single-phase composite within the scopeof this invention is based on the idea that the melting temperature ofthe crystals in suitable solvent is significantly lower than the meltingpoint of the semicrystalline polymer proper. In the case of UHMWPE, thecrystals in the pseudo-gel of UHMWPE in paraffin oil melt atapproximately 120° C., depending on the preparation conditions, whereasa semicrystalline morphology of the same polymer obtained from thegel-precursor (after solvent extraction) has a melting point of130°-137° C. and even higher, above 140° C., after its drawing into afibrillar structure.

Thus, the UHMWPE pseudo-gel can be applied in a temperature range below130° C. on UHMWPE drawn fibers without affecting the mechanicalperformance of the fibers The processing temperature for the singlephase composite can be extended to lower limits (100° C. or lower),depending on the concentration and the preparation conditions of thepseudo-gel state; however, it is preferred that the temperature be closeto or above the melting temperature of the crystals in the pseudo-gel,so that the latter can flow more readily and wet the reinforcing fibersfor better bonding between the matrix and the fibers. Upon cooling to˜120° C., the pseudo-gel is formed onto the reinforcing fibers which, asdiscussed in the parent patent is comprised of a continuous molecularnetwork of crystals. After solvent extraction or evaporation, thecomposite is comprised of only the residual semicrystalline UHMWPEmatrix on the UHMWPE fibers. This process is substantially differentfrom the melt processing of the prior art for fabricating compositestructures. For example, in the melt-process, the matrix is a solid (nosolvent(s) is present) which is heated to melt flow around thereinforcing fibers (in contrast to forming a solution in our case), andsubsequently cooled to crystallize onto the fibers by meltcrystallization (instead by gelation as the present process). Also, thedifferent from the "solid-like" pseudo-gel behavior of the matrix of thepresent process.

The reinforcing fibers can be continuous in some particular direction,dispersed randomly, and knitted or woven in some particular pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing a deposited layer of UHMWPEmatrix on the crystalline reinforcing UHMWPE fibers. The magnificationis 10,000x, and the dimension of one micrometer is shown by the whitebar.

FIG. 2 is an electron micrograph showing "shish-kebab" crystals of theUHMWPE matrix onto the crystalline lamellar structure of the UHMWPEreinforcing fibers The magnification is 15,000x, and the dimension of amicrometer is shown by the white bar.

FIG. 3 is an electron micrograph showing two adjacent and aligned UHMWPEreinforcing fibers with their characteristic crystalline lamellarstructure partially covered by the UHMWPE matrix. The magnification is5,000x, and the dimension of one micrometer is shown by the white bar.

FIG. 4 is an exploded, diagrammatic view in perspective of a compositelaminated structure embodying the principle of this invention and havinga single knitted fibrillar structure between two pseudo-gel sheets.

FIG. 5 is a similar view of a multilayer composite structure accordingto this invention.

FIG. 6 is a flow sheet with diagrammatic views of each step in a processembodying the invention for making a tubular composite product.

FIG. 7 is another flow sheet with diagrammatic views illustrating aprocess of the invention for making a flat structure.

DESCRIPTION OF SOME PREFERRED EMBODIMENTS

Single-phase composites of this invention can be made by using anultra-high-molecular-weight polymer such as UHMWPE and applying it as apseudo-gel in a volatile (e.g., Decalin) or non-volatile (e.g., paraffinoil) solvent on a single- or plural-layer fibrillar structure (such ashigh-modulus high-strength fibers or ribbons laid in orderly manner ordispersed randomly), of the same polymer resin (e.g., UHMWPE) or frompolymer resins with identical or similar properties prior to theirprocessing into the matrix and fiber forms, in a temperature range ingeneral between room temperature and the melting point of the UHMWPEhigh-modulus fibers or ribbons of approximately 140°-145° C. (andpreferably between the dissolution temperature of the crystallinemorphologies (about 120° C.) in the UHMWPE pseudo-gel and 130° C.), andsubsequently cooling the composite system to below the gelationtemperature of the pseudo-gel (approximately 120° C.) under compressionof approximately 1000-5000 psi. When applying the pseudo-gel attemperatures above 130° C., the filaments should be under tension, toprevent shrinkage of the fibers.

Unlike the prior-art transcrystallization process which involves thecrystal growth of the matrix upon its melt crystallization perpendicularto the direction of the reinforcing fibers, the present invention usesgelation of the matrix on the fibers that involves the generation ofrandomly oriented stacks of single crystals and "shish kebab" crystalsand their deposition onto and around the crystalline reinforcing fibers.

Thus, FIG. 1 shows the UHMWPE matrix deposited onto the crystallinereinforcing fibers. The fibers have a crystalline lamellar structureperpendicular to the fiber direction.

FIG. 2 shows shish-kebab crystals of the UHMWPE matrix deposited ontothe crystalline lamellar structure perpendicular to the direction of thereinforcing fibers.

FIG. 3 shows two adjacent and aligned reinforcing fibers partiallycoated with UHMWPE matrix.

The composite may be a flat sheet, a tube, or a solid rod.

Thus, FIG. 4 shows two UHMWPE pseudo-gel sheets 10 and 11 with a knittedfibrillar structure 12 sandwiched between them. The fibers of thestructure 12 are also made of UHMWPE.

FIG. 5 similarly shows, exploded, a multilayer composite structure madeup of four UHMWPE pseudo-gel sheets 15, 16, 17, and 18 with threeknitted fibrillar structures 20, 21, and 22 successively sandwichedbetween them.

The compression process can take place in a continuous or non-continuousprocess, as practiced in a rolling or compression molding, and otheralternatives are also available. Subsequently, the solvent can beremoved from the matrix of the composite by simple evaporation, in thecase of the volatile solvent, such as Decalin (decahydronaphthalene), orextraction with a suitable solvent (e.g., hexane) in the case of thenon-volatile solvent (paraffin oil). The reinforcing fibers in thecomposite can be continuous or short, and can be knitted, woven,randomly dispersed or otherwise used.

The so obtained "UHMWPE gel coated" fibrillar reinforcements of UHMWPEcan be prepared in single or more layers by superimposing the singlelayers and compressing the entire assembly into a single or a multilayercomposite structure and then evaporating the volatile solvent orextracting the non-volatile solvent with a suitable solvent as in thecase of the single layer composites. Alternatively, the UHMWPE fibrillarreinforcements can be commingled in the UHMWPE pseudo-gel so that whenthe solvent is removed (either by evaporation or extraction depending onthe solvent) a single phase composite of randomly dispersed UHMWPEfibers in UHMWPE matrix is obtained.

Other polymers that may be processed into such one-phase compositeproducts in which the matrix and the reinforcing fibers are made of thesame polymer, as defined in this specification, include isotacticpolypropylene, poly(ethylene terephthalate), polyamides, poly(vinylalcohol), polyacrylonitrile, and other linear groups in their chainbackbone so that they can form a , pseudo-gel state of the typedescribed in the mother patent and capable of deforming intohigh-modulus, form, these linear polymers may be as single-strandfibers, multi-strand fibers, or ribbons. The filaments may be long orchopped, randomly dispersed or texturized, knitted, woven, or braided.

EXAMPLE 1

In one experiment, a UHMWPE (Hercules HiFax 1900, M_(w) ≧3.2×10⁶) gel(5% UHMWPE in 95% paraffin oil, by weight) was heated to 125° C. andapplied to a woven UHMWPE (Hercules HiFax 1900, M_(w) ≧3.2×10⁶)high-modulusstrength-structure on either side. The so coated wovenstructure was then placed under compression at about 2,000 psi at 125°C. The temperature was lowered subsequently to at least about 100° C.,and the gel-coated UHMWPE knitted structure with high modulus and highstrength was retrieved. This composite system was then placed in anextractor to extract the non-volatile paraffin oil with hexane, whichwas removed from the composite by evaporation to obtain the single phasecomposite of woven UHMWPE fibers in UHMWPE matrix.

EXAMPLE 2

In an independent experiment, a UHMWPE (American Hoescht-Celanese,Hostalen GUR-412, M_(w) ˜3.3×10⁶) gel (5% UHMWPE in 95% paraffin oil, byweight) was heated to 125° C. and applied to a woven UHMWPE (HerculesHiFax 1900, M_(w) ≧3.2×10⁶) high-modulus, high-strength structure oneither side. The so-coated woven structure was processed under the sameprocedure as in Example 1 to obtain a single-phase composite of wovenUHMWPE fibers in a UHMWPE matrix of different UHMWPE origin.

EXAMPLE 3

In another independent experiment, a UHMWPE (American Hoescht-Celanese,Hostalen GUR-412, M_(w) ˜3.3× 10⁶) gel (5% UHMWPE in 95% paraffin oil)was heated to 125° C. and applied to UHMWPE (Mitsubishi Hizex Million240M, M_(v) =2×10⁶) high-modulus, high-strength fibers which werealigned uniaxially. The so coated uniaxial structure was processed underthe same procedure as in Example 1 to obtain a single phase composite ofuniaxially aligned UHMWPE fibers in a UHMWPE of different UHMWPE origin.

EXAMPLE 4

The same procedure was followed with the multilayer composite structure.The UHMWPE (Hercules HiFax 1900, M_(w) ≧3.2×10⁶) gel-coated UHMWPE(Hercules HiFax 1900, M_(w) ≧3.2×10⁶) woven layers were superimposed andthe assembly was compressed and subsequently treated as described abovefor the single-layer structure in Example 1.

EXAMPLE 5

In one particular experiment, an UHMWPE (Hercules HiFax 1900, m_(w)≧3.2×10⁶) paraffin oil pseudo-gel (4% w/w) was heated to 118° C. (atthis temperature the pseudo-gel turned into a viscous fluid) and wasapplied on a woven UHMWPE (Hercules HiFax 1900, M_(w) ≧3.2×10⁶)fibrillar structure by merging the latter into a thin may be coated onone or both sides depending on the spacing between the knitted fibrils.)Subsequently, the and, in another, passed between compression rolls andcooled until the viscous coating became an opaque pseudo-gel (at T≧118°C.). Thereafter, the solvent was removed from the pseudo-gel coating bythe extraction methodology described above for the single-layerstructure.

EXAMPLE 6

In another experiment, a UHMWPE (American Hoescht-Celanese, HostalenGUR-412, M_(w) ˜3.3×10⁶) gel (5% w/w UHMWPE in paraffin oil) was heatedto 125° C. Then, randomly oriented and dispersed UHMWPE fibers (HerculesHiFax 1900, M_(w) ≧3.2×10⁶) were commingled in it and the system wasplaced under compression at about 2000 psi at 125° C. The temperaturewas decreased subsequently to at least about 100° C. and a UHMWPEgel-coated "mesh" of randomly dispersed high-modulus and high-strengthfibers of UHMWPE was obtained. This system was then placed in anextractor to extract the paraffin oil with hexane, which was removedfrom the composite by evaporation to obtain a single phase composite ofrandomly mixed UHMWPE high modulus and strength fibers in UHMWPE.

EXAMPLE 7

For a multilayer composite structure, for example, a composite structurewith five layers of UHMWPE woven fibrillar structures, the coated UHMWPEwoven fibrils with UHMWPE pseudo-gel were superimposed and compressionmolded at 120° C. The multilayer composite systems were then placed inan extractor to extract the non-volatile paraffin oil with hexane, whichwas removed from the composite by evaporation.

EXAMPLE 8

An alternative methodology for the preparation of the single-phasecomposites involves the stacking of thin fibrillar layers for thepreparation of multilayer composites or the use of the thin pseudo-gelfilms to "sandwich" a single UHMWPE knitted fibrillar layer for thepreparation of a composite with a single fibrillar layer as shown inFIG. 4. The thin UHMWPE pseudo-gel films were obtained according to theprocessing methodology involving the first two steps of FIG. 6, in whichpieces 25 of the UHMWPE pseudo-gel (UHMWPE/paraffin oil 5% weight, byweight) are compressed at 26 to make a gel-like sheet 27. The assembliesof the thin gel film and knitted structure(s) were compression molded atapproximately 123° C. and retrieved after cooling to below 120° C. forthe removal of the non-volatile solvent as mentioned above.

EXAMPLE 9

A tubular composite structure with a "sandwiched" UHMWPE fibrillar layerbetween thin UHMWPE pseudo-gel films were obtained by the process ofFIG. 6. Pieces 25 of the pseudo-gel are compressed at step 26 to make agel-like sheet 27. The sheet 27 is wrapped around a mandrel 28 toproduce a gel-like tube 30. Then a tubular UHMWPE fibrillar woven orknitted structure 31 is passed sheet 32 is wrapped on top of the UHMWPEfibrillar woven or knitted structure 31, and the assembly is compressionrolled against an adjacent rolling surface 33 of a roller 34 atapproximately 120° C. Depending on the size of the interstices ortexturization of the weaving or knit pattern, the outside UHMWPEgel-sheet 32 may not be required, as the inner layer 30 may exudethrough the fibrils of the middle woven or knitted layer 31 and thusallow for a uniform coating on both sides. After cooling the so-preparedtubular samples to below 120° C., they were retrieved for the removal ofthe non-volatile solvent as described above, to give a tubular product35.

EXAMPLE 10

Alternative methodologies involving the continuous UHMWPE fibrillarstructure can be used also. FIG. 7 is a schematic of a continuousprocess for the preparation of single-phase sheet composites of UHMWPE.A knitted or woven fibrillar structure 40 is fed from a mandrel 41 to agel-coating zone 42. The resulting gel coated fibrillar structure 43 ispassed through between cooling and compression rolls 44 and 45 (aplurality of rolls may be used also) to form a composite 46, which afterthe extraction 47 of the non-volatile solvent gives a flat compositestructure 48. Such a composite structure 48 can be cut to size, treatedthermally and/or under compression in step 49 to a final product 50.

Such systems (single-phase composites) may combine the physicalproperties, e.g., transport properties of the isotropic matrix and thehigh mechanical performance of the same polymer with a fibrillarstructure, e.g., in knitted Dacron arterial prostheses, the porosity ofthe tubular knitted structure can be controlled beyond the presentlimits of adjustment by thread size or interstices size or texturizationof the knit pattern, by the concentration and thickness of thepoly(ethylene terephthalate) pseudo-gel coating on the knitted tubularprosthesis.

Similarly, in the UHMWPE "arterial" prostheses based on a knitted UHMWPEtubular structure and a pseudo-gel coating of the parent and this patentapplication, the porosity of the arterial prostheses can be controlledby the concentration and thickness of the UHMWPE pseudo-gel coating onthe knitted tubular device and by adjustment of the thread size orinterstices size or texturization of the knitted pattern.

EXAMPLE 11

Another methodology of continuous production of a single-phase compositeof UHMWPE in sheet form involves the formation of a gel-coated mesh ofrandomly dispersed and oriented UHMWPE fibers by commingling the UHMWPEfibers in a UHMWPE pseudo-gel heated to a temperature close to but aboveits gelation temperature and its subsequent cooling and compressionthrough a set of cooling and compression rolls. This composite meshstructure is then taken to an extraction step to remove the paraffin oilwith hexane which is removed from the composite by evaporation. Theproduct is a single-phase composite sheet comprised of a mesh of UHMWPEhigh-modulus and high-strength fibers in UHMWPE.

Similar structural configurations of single phase composites can beobtained with other polymers such as polypropylene, poly(vinyl alcohol),polyamides, and polyesters.

Isotactic polypropylene (M_(v) ˜3.4×10⁶) can be dissolved in decalin(for example, 1% w/v) at approximately 185° C. in the presence of anantioxidant such as Irgonox-1076 (Ciba-Geigy Co.) (0.5% w/w of thepolymer) and form a pseudo gel state by cooling to a temperature of ≦90°C. Such a pseudo-gel can be applied onto high-modulus and high-strengthfibers of isotactic polypropylene produced by spinning or solid statedeformation processes, by the processing methodology described above toobtain a structure of polypropylene coated polypropylene fibers. Thesolvent can be removed from these structures to some extent bycompression or by vacuum removal. The fibers can be in the form ofmonofilaments, they can be long or short and in knitted, woven, braided,or randomly dispersed structures. The dried composite can be processedfurther, as described above.

Alternatively, isotactic polypropylene can be dissolved in paraffin oil(for example, 6% w/w) at about 200° C. and form a pseudo-gel by coolingto ≦90° C. In this case (i.e., when a non-volatile solvent is used)after the polypropylene is applied onto the polypropylene fibers, theparaffin oil can be extracted with a solvent such astrichlorotrifluroethane which subsequently can be removed byevaporation.

Poly(vinyl alcohol) (M_(w) =150,000) can be dissolved in a 2:1 ethyleneglycol and water mixture (for example 4% w/w) at 135° C. and form apseudo-gel by cooling to ambient temperature The poly(vinyl alcohol) canbe applied onto poly(vinyl alcohol) high-modulus and high-strengthstructure as described above. The solvent system can be removed from thecomposite by compression or vacuum removal Again, the poly(vinylalcohol) high-modulus and high-strength fibers can be in different formsand patterns or be randomly dispersed as described above.

Nylon 6 (M_(v) ˜180,000) can be dissolved in benzyl alcohol (for example5% w/w) at approximately 165° C. and form a pseudo-gel on cooling toambient temperature. The Nylon 6 pseudo-gel can be applied ontomelt-spun Nylon 6 fibers, as described above. The Nylon 6 fibers in thesingle-phase composite can be in different forms and patterns orrandomly dispersed. The solvent can be removed from the composite bycompression and/or vacuum removal.

Poly(ethylene terephthalate) (M_(v) ˜100,000) can be dissolved innitrobenzene (for example, 10% w/w) and form a pseudo-gel on cooling toambient temperature. Such a pseudo-gel can be applied onto melt spun orotherwise produced poly(ethylene terephthalate) high-modulus andhigh-strength fibers to form a single phase composite : using theprocess described above.

EXAMPLE 12

A methodology for the preparation of single-phase composite in the formof solid rods, involves the immersion of a braided UHMWPE structure ofhigh-modulus and high-strength fibers into a UHMWPE pseudo-gel heated toa temperature close to but above its gelation temperature and itssubsequent cooling and compression :, through a converging die geometryor compression mold. The composite is then taken to an extraction stepand/or evaporation step as described above to obtain a single phasecomposite using a braided UHMWPE structure of high-modulus andhigh-strength fibers.

EXAMPLE 13

An alternative methodology for the preparation of a single phasecomposite in the form of a solid rod uses the same steps described forthe sheet laminate structures apart that the laminates of the UHMWPEpseudo-gel and the UHMWPE knitted or woven structures are rolled tightlyand compressed into a solid rod which subsequently is taken to anextraction and/or evaporation step, as described above, to obtain asingle-phase composite having a circular laminated structure.

Since, when making a solid rod with a thick cross-sectional area usingthe methodology explained above, it is time consuming to remove anon-volatile solvent by extraction, the use of a volatile solvent ispreferable, for it can be removed more readily by evaporation.

In addition to the biomedical uses suggested in this patent applicationthere are numerous other applications, which include tendons, ligaments,porous membranes, screens, high pressure vessels and pipes, ballisticapplications and structural components.

To those skilled in the art to which this invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. Such embodiments may includethe fabrication of single-phase composite systems in which the fibersare dispersed randomly or laid in some particular pattern, thefabrication of single composites in which the fibers are short or long,the fabrication of composites in which the polymer matrix which iscapable of gelling and the polymer fiber reinforcements are compatible,the fabrication of composites in which the polymer matrix is filled withadditives or blended with compatible polymers for the purpose of e.g.,enhancing its adhesion, and the fabrication of laminate structuresincorporating such single phase polymer composites and metal sheets.

A few examples of such embodiments are the fabrication of a compositestructure comprised of, e.g., an isotactic polypropylene matrix andUHMWPE fibers. The isotactic polypropylene forms in decalin a pseudo-gelby cooling to ≦90° C. and hence it can be applied on the UHMWPE fiberswithout affecting its mechanical properties. Similarly, a polyamideforms a pseudo-gel (e.g. Nylon 6 in nitrobenzene) on cooling to ambienttemperature and can be applied onto UHMWPE fibers. The disclosures andthe descriptions herein are purely illustrative and are not intended tobe in any sense limiting.

What is claimed is:
 1. A method of making a single phase compositeproduct, comprising the steps of:heating a sheet of UHMWPE pseudo-gelcomprising 5% UHMWPE in 95% paraffin oil, by weight, to 125° C., so thatthe pseudo-gel dissolves into a solution, applying a knitted UHMWPE highmodulus, high-strength fibrillar structure on one side thereof, saidfibrillar structure being formed from a semicrystalline linear polymerwhich is the same as said pseudo-gel, with a weight average molecularweight of at least 500,000, placing the combined layers undercompression at about 2,000 psi at about 125° C. to form a unitarycomposite structure, lowering the temperature to at least about 100° C.,to give a gel-coated UHMWPE knitted structure with high modulus andstrength, removing the paraffin oil from the gel coated structure byextraction with hexane, and removing the hexane form the gel componentof the composite system by evaporation, to obtain a single phasecomposite of UHMWPE fibers in UHMWPE matrix.
 2. The method of claim 1having the steps of:coating the plurality of UHMWPE woven, knitted, orrandomly crossed layers with a plurality of pseudo gel-UHMWPE sheet-likelayers before compressing the composite.
 3. A method for making asingle-phase multilayer composite comprising the steps of:heating anUHMWPE/paraffin oil pseudo-gel (4% w/w) to about 118° C. so that thepseudo-gel dissolves into a solution and flows like a viscous fluid,applying over that a woven, knitted or randomly crossing UHMWPEfibrillar structure, said fibrillar structure being formed from asemicrystalline linear polymer which is the same as said pseudo-gel,with a weight average molecular weight of at least 500,000. merging thefibrillar structure into a thin layer of the pseudo-gel, compressing theresulting structure, to form a unitary composite structure cooling ituntil the pseudo-gel coating becomes opaque, and removing the paraffinoil from the pseudo-gel.
 4. A method for making a single-phase compositestructure, comprising the steps of:interlocking sheet-like pseudo-gelstructures of UHMWPE incorporated with solvent with a series of layersof UHMWPE woven, knitted, or randomly crossing fibrillar structures,said pseudo-gel and said fibrillar structures being formed form the samelinear polymer resin, compression molding the interleaved structure at120° C. so that the pseudo-gel sheet-like structures dissolve into asolution at a temperature substantially lower than the meltingtemperature of about 140° C. of the UHMWPE fibrillar structure and flowaround and in between said fibrillar structure to form a unitarycomposite structure, cooling it to below 120° C. so that the solutionbecomes again a pseudo-gel, and removing the solvent originallyincorporated in the pseudo-gel structures form the composite structure.5. A method for making a single-phase composite comprising the stepsof:stacking thin UHMWPE pseudo-gel films incorporating a solvent betweenalternating UHMWPE fibrillar layers, said pseudo-gel films and saidfibrillar layers being formed from the same linear polymer material,compression molding the resulting structure at approximately 123° C. sothat the pseudo-gel films dissolve into a solution at a temperaturesubstantially lower than the melting temperature of about 140° C. of theUHMWPE fibrillar layers and flow around, and in between said fibrillarlayers, cooling it to below 120° C. so that the solution becomes again apseudo-gel, and removing the solvent originally incorporated in thepseudo-gel films to form a unitary composite structure.
 6. A method formaking a single-phase composite structure comprising the stepsof:compressing pieces of UHMWPE pseudo-gel incorporating a solvent tomake a gel-like sheet, wrapping the sheet around a mandrel, to produce agel-like tube, wrapping a tubular UHMWPE fibrillar knitted structurearound said sheet, said fibrillar structure being formed from asemicrystalline linear polymer which is the same as said pseudo-gel,with a weight average molecular weight of at least 500,000, wrapping nadditional gel-like sheet like the first on top of the UHMWPE fibrillarknitted structure, compression rolling the resulting tube against anadjacent roller surface at approximately 120° C. so that the gel-likesheet dissolves into a solution at a temperature substantially lowerthan the melting temperature of about 140° C. of the UHMWPE fibrillarstructure and flows around and in between the said fibrillar structure,cooling the so-prepared tubular structure to below the 120° C. so thatthe solution becomes again a pseudo-gel, and removing the solventtherefrom to give a unitary composite tubular product.
 7. A method forcontinuously making a single phase sheet composite, comprising the stepsof:feeding a fibrillar structure from a mandrel to a coating zone,coating the fibrillar structure with a pseudo-gel incorporated with anon-volatile solvent at a temperature so that the pseudo-gel dissolvesinto a solution and which is substantially lower than the meltingtemperature of the fibrillar structure, said fibrillar structure beingformed from a semicrystalline linear polymer which is the same as theone in said pseudo-gel, with a weight average molecular weight of atleast 500,000, pressing the resulting pseudo-gel coated fibrillarstructure between cooling and compression rolls, to form a pseudo-gelcoated sheet, cutting the sheet to size, compression molding the sheetat approximately 120° C. so that the gel-like sheet dissolves into asolution at a temperature substantially lower than the meltingtemperature of the fibrillar structure and flows around and in betweensaid fibrillar structure to form a composite, and extracting thenon-volatile solvent form the composite to give a flat unitary compositestructure.
 8. A method for making a single-phase composite product,comprising the steps of:heating a sheet of a pseudo-gel of a linearpolymer consisting of:(1) ultra-high-molecular-weight polyethylene inthe solvent paraffin oil, (2) isotactic polypropylene in the solventparaffin oil or Decalin, (3) poly(L-lactide) in the solvent chloroform,(4) poly (vinyl alcohol) in the solvent ethylene glycol and water, (5)polyacrylonitrile int he solvent methyl formamide or tetramethylenesulfone, (6) poly(ethylene terephthalate) in the solvent nitro benzene,or (7) a polyamide in the solvent benzyl alcohol, at the meltingtemperature for said pseudo-gel to form a dissolved solution which flowslike a viscous fluid, applying a high modulus, high-strength fibrillarstructure of the same polymer to one side thereof, placing the combinedlayers under compression at about 2,000 psi at said melting temperatureto form a unitary composite structure, lowering the temperature to atleast about 100° C., to give a gel-coated composite structure with highmodulus and strength, removing any solvent from the gel coated structureby extraction with a more volatile solvent, and removing the morevolatile solvent from the pseudo-gel component of the composite systemby evaporation, to obtain a single phase composite of fibers in matrix.